Rechargeable electrochemical cells using a hydrogen storage alloy as the active material for the negative electrode are known in the art. The negative electrode is capable of the reversible electrochemical storage of hydrogen. The positive electrode typically comprises a nickel hydroxide active material although other active materials, such as manganese hydroxide, may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte. A suitable separator (i.e., a membrane) may also be positioned between the electrodes. As used herein the terminology “metal hydride material”, “hydrogen storage alloy”, and “hydrogen absorbing alloy” are synonymous.
Upon application of an electrical current to the negative electrode, the active metal hydride material is charged by the absorption of hydrogen. This is shown by reaction (1).M+H2O+e−→M−H+OH−(Charging)  (1)
Upon discharge, the stored hydrogen is released by the metal hydride material to provide an electric current. This is shown by reaction (2).M−H+OH−→M+H2O+e−(Discharging)  (2)
The reactions at a conventional nickel hydroxide positive electrode as utilized in a nickel-metal hydride electrochemical cell are as follows:Ni(OH)2+OH−→NiOOH+H2O+e−(Charging)  (3)NiOOH+H2O+e−→Ni(OH)2+OH−(Discharging)  (4)
Based on the pioneering principles of Stanford R. Ovshinsky, a family of extremely efficient electrochemical hydrogen storage materials were formulated. These are the Ti—V—Zr—Ni type active materials such as those disclosed in U.S. Pat. No. 4,551,400 (“the '400 Patent”) the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti—V—Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al.
Other examples of metal hydride alloys are provided in U.S. Pat. No. 4,728,586 (“the '586 Patent”) the disclosure of which is incorporated herein by reference. The '586 Patent describes a specific sub-class of these Ti—V—Ni—Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Still other examples of hydrogen absorbing alloys are provided in U.S. Pat. No. 5,536,591 (“the '591 Patent”), the disclosure of which is incorporated herein by reference. In particular, the '591 Patent provides teaching on the type of surface interface at the metal hydride electrode and the nature of catalytic sites ideal for promoting high rate discharge.
In part, due to the research into the negative electrode active materials, the Ovonic nickel-metal hydride batteries have demonstrated excellent performance characteristics such as power, capacity, charging efficiency, rate capability and cycle life. Presently, there is an increasing use of rechargeable nickel-metal hydride batteries in all types of portable tools, appliances, and computer devices. As well, there is a growing use of nickel-metal hydride cells in applications such as electric and hybrid-electric vehicles. Many of the new uses for the nickel-metal hydride cells require that further improvements be made in the cell's performance.
Many of the performance characteristics of a nickel-metal hydride cell are affected by the surface conditions of the active metal hydride material used in the cell's negative electrode. For example, the power of the cell is affected by both the surface composition and the surface area of the metal hydride material. The appropriate modification of the surface composition and/or the surface area can change the surface kinetics of the hydride reaction so as to lower the charge transfer resistance of the material.
Hydrogen storage alloys are sensitive to the formation of oxides and the alloy surfaces comprise, to a great extent, metal oxides. The composition of these oxides depends on many factors including the composition, morphology and method of preparation of the hydrogen storage alloy. Generally, the type of surface oxides which form naturally and not by design may be detrimental to the performance of the negative electrode and cell. Oxides at the surface of the hydrogen absorbing alloy decreases the alloy's catalytic (charge transfer) capabilities, thereby decreasing both the charging and discharging efficiency of the electrode and cell.
During cell charging, the decreased surface kinetics of the alloy shifts the potential at the surface of the electrode so as to increase the evolution of hydrogen gas via the hydrogen evolution reaction:2H2O+2e−→H2+2OH−  (5)
Atomic hydrogen formed at the surface of the negative electrode can either recombine with another atomic hydrogen and escape as molecular hydrogen gas or it can react with the hydrogen absorbing alloy in the electrode to form a hydride. If the surface of the hydrogen absorbing alloy is covered with oxides, hydride formation is inhibited and hydrogen evolution is preferred. Electric current (e.g., electrons) applied to the negative electrode for the purpose of charging the electrode via charging reaction (1) is instead “wasted” in the production of hydrogen gas via reaction (5). This decreases the charging efficiency of the cell and increases the pressure of hydrogen gas within the cell. The decreased surface kinetics also increases the charge transfer resistance of the material and the electrode so that more power is wasted due to internal dissipation. It is also believed that the surface oxides polarize the electrode so as to reduce the rate at which the cell discharge process proceeds.
Many of the surface oxides are very dense and impermeable to hydrogen transfer thereby increasing the resistance to hydrogen diffusion during both the charging and discharging processes. This has a detrimental effect on the rate capability of the electrode.
U.S. Pat. No. 4,716,088, the contents of which is incorporated by reference herein, describes a method of “activating” the hydrogen storage alloy material by immersing the material into a alkaline solution. This “alkaline etch treatment” modifies the composition and morphology of the alloy surface so as to improve the electrochemical activity of the alloy and the electrodes formed from the alloy.
The activation process modifies the composition of the oxide layer on the surface of the alloy. The oxide composition depends upon the composition of the underlying hydrogen storage alloy as well as the corrosivity of the different metals which form the alloy. Certain metals such as titanium, zirconium and manganese have a greater affinity for oxidation while other metals such as nickel do not oxidize as readily. Oxide composition may also depend upon the specific process used to make the alloy since certain processes may promote oxidation more than others.
It is believed that immersing the hydrogen storage alloy into the alkaline solution at least partially dissolves certain oxides from the alloy surface. The extent of dissolution depends upon the solubility of the specific oxide in the alkaline environment. Certain oxides, such as oxides of manganese, vanadium, aluminum and cobalt are readily soluble in an alkaline solution while others, such as those of titanium, zirconium and nickel are less soluble.
The alkaline etch treatment modifies the oxide composition of the alloy surface so as to increase the catalytic activity (charge transfer capabilities) of the material. While not wishing to be bound by theory, it is believed that the activation process increases the concentration of nickel metal at or near the alloy's surface. Increasing the catalytic activity of the alloy surface lowers the charge transfer resistance of the material and electrode. The lowered resistance results in more efficient battery discharge since there is less power wasted due to internal dissipation and more power available for battery output. The lowered resistance also increases the charging efficiency of the cell since it shifts the voltage on the surface of the negative electrode away from the hydrogen evolution potential.
Activation also provides for a “gradual transitioning” in the composition and/or oxidation state of the oxide layer from the electrolyte/oxide interface to the bulk material. For example, the oxide layer after activation may have a small concentration of soluble components near the electrolyte interface but a composition more closely resembling the bulk material further away from the interface. This “gradient-type” surface may have an electrical and catalytic nature which is more suitable for electrochemical charging and discharging.
The activation process disclosed in the '088 Patent describes an alkaline etch treatment wherein the temperature of the alkaline solution as well as the time in which the hydrogen storage alloy is left in contact with the alkaline solution are both variables that affect the results of the process. The present invention describes an alkaline etch treatment of a hydrogen absorbing alloy and an alkaline etch treatment of a hydrogen absorbing alloy electrode wherein the concentration of the alkaline solution is also a result-effective variable which can be varied to provide an activated hydrogen storage alloy and an activated hydrogen storage alloy electrode with increased surface area and improved electrochemical properties.